1. Field of the Invention
The present invention relates to a light-emitting device which performs wavelength conversion with the use of a fluorescent material.
2. Description of the Related Art
A light-emitting diode is practically used in industrial products as a compact and lightweight light source which emits light with high efficiency. Light emission from the light-emitting diode, however, has a narrow spectral width, and hence it is difficult impossible to obtain white light from a single device. Accordingly, a light-emitting device for obtaining white light with a wide spectral width by mixing blue light and yellow light has been proposed (for example, Japanese Patent Laid-Open Publication No. 2000-208815), and is practically used. The light-emitting device includes a blue light-emitting diode and a fluorescent material for absorbing blue light and for emitting yellow light.
The following can be used for the fluorescent material for emitting yellow light: an yttrium aluminum garnet fluorescent material activated by Ce (Y3Al5O12:Ce, (Y,Gd)3Al5O12:Ce, or the like); a terbium aluminum garnet fluorescent material activated by Ce (Tb3Al5O12:Ce or the like); an alkaline earth metal orthosilicate fluorescent material activated by Eu ((Sr,Ca,Ba)SiO4:Eu or the like), or the like. These materials absorb light emission from the blue light-emitting diode, and emit yellowish fluorescent light.
To manufacture these fluorescent materials, oxide particles and/or metallic salt are/is combined and mixed in accordance with the composition ratio of a desired fluorescent material. A mixture is put into a crucible made of ceramic with flux or the like. The mixture and the flux are subjected to heat treatment at high temperatures in an atmospheric atmosphere or a reduction atmosphere, to prepare a fluorescent material with desired composition by baking. According to this method, the diameter of crystal grains of the fluorescent material grows to several μm or more by baking. The resulting grains of the fluorescent material become a material for a light-emitting diode, after being cleaned, dried, and sized by crushing and by a sieve that evens their diameters. This method makes it possible to manufacture the fluorescent material in quantity. The manufactured fluorescent material is dispersed in a transparent resin, and the resin is charged into a case in which the blue light-emitting diode is disposed. Thus, a white light-emitting diode is manufactured.
The fluorescent material used in the conventional white light-emitting diode has extremely high light conversion efficiency at ambient temperature. There is a problem, however, in that the light conversion efficiency decreases at high temperatures. In this case, if the electric current density of the blue light-emitting diode is increased to increase the intensity of light emission, the light emission efficiency of the fluorescent material is decreased due to heat from the blue light-emitting diode. Thus, yellow light is reduced, so that the color of light emitted from the diode is bluish white. In a like manner, the color of emitted light changes when the ambient temperature is increased. As described above, reduction in the light emission efficiency of the fluorescent material in an environment at high temperatures is a phenomenon called “temperature quenching,” and there was no effective method for resolving it.
In the conventional white light-emitting diode, there were cases where the density of the fluorescent material charged into the case was biased in the resin. Thus, there were problems of unevenness in luminance and unevenness in the color of emitted light. The unevenness in the color of emitted light was a critical defect in applying the white light-emitting diode to a backlighting source for liquid crystal display. When a product emitted a color of light that was out of a predetermined chromaticity range, the product was rejected as defective. Therefore, product yield was low and there was much waste.